<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 200px;
            outline: 1px solid black;
        }
        @keyframes identifier {
            
        }
        .circle{
            width: 100px;
            height: 100px;
            box-sizing: border-box;
            border: 1px solid black;
            border-radius: 50%;    
            animation: name duration timing-function delay iteration-count direction fill-mode;      

        }
    </style>
</head>
<body>
    <div class="box">
        <div class="#circle"></div>
    </div>
    1.优先级
    2.动画
    3.弹性布局
    4.border-radius
   <div class="box">
       <div class="circle"></div>
   </div>
</body>
</html>